Polyrhodanine photoconductive materials

ABSTRACT

New compositions comprising polyrhodanines with three or more rhodanine rings are described. These compositions can be used as pigment-type photoconductors by dispersing them in insulating binders, or as dyes.

United States Patent 11 1 Keller et al.

[ 1 Oct. 14,1975

[ POLYRHODANINE PHOTOCONDUCTIVE MATERIALS [75] Inventors: Juergen H. H.Keller, Chelmsford;

Robert H. Sprague, Carlisle, both of 21 Appl. No.: 366,910

[52] US. Cl. 96/l.5; 96/1.6; 96/1.7; 96/139; 260/3067 R [51] Int. Cl.G03G 5/06 [58] Field of Search 96/1.5, 1.6, 1.7

[56] References Cited UNITED STATES PATENTS 3,047,384 7/1962 Jones eta1. 96/l.7 3,110,591 11/1963 Stewart 96/1.7 3,140,283 7/1964 Depoarteret 96/1.5 X 3,155,503 11/1964 Cassiers et a1. 96/l.5 3,507,649 4/1970Hensley 96/l.7 3,507,692 4/1970 Ghys et al.. 96/1.5 X 3,563,735 2/1971Emmi 96/1.7 3,647,433 3/1972 Contais 96/1.5 X 3,658,522 4/1972 Endo eta1. 96/1.6 X 3,687,946 8/1972 Yao 96/1.6 X 3,719,480 3/1973 Brantly96/1.5 X 3,743,638 7/1973 Webster et a1... 96/1.7 X 3,748,128 7/1973McNally 96/1.5 X 3,759,931 9/1973 Brown et a1. 96/1.7 X 3,772,28111/1973 Tubuko et a1. 96/l.6 X

3,782,935 1/1974 Yao 96/1.6 X 3,796,573 3/1974 Jones 3,839,327 10/1974Yao 96/1.5 X

FOREIGN PATENTS OR APPLICATIONS 45-32756 10/1970 Japan 96/1.6

964,875 7/1964 United Kingdom.... 964,877 7/ 1964 United Kingdom....1,226,892 3/1971 United Kingdom....

45-2879 1/1970 Japan 96/1.7

OTHER PUBLICATIONS Dye-sensitized Photoconductive Compositions, 10015,Research Disclosures, Product Licensing 1ndex, Aug. 1972, pp. 4951.

Sprague et a1., Dye Sensitization of Photosensitive Titanium Dioxide,Photographic Science & Engineering, Vol. 14, No. 6, Nov.-Dec., 1970, pp.401-406.

Primary Examiner-Roland E. Martin, Jr.

Assistant Examiner-John R. Miller Attorney, Agent, or Firm-Homer 0.Blair; Robert L. Nathans; David E. Brook [57] ABSTRACT New compositionscomprising polyrhodanines with three or more rhodanine rings aredescribed. These compositions can be used as pigment-typephotoconductors by dispersing them in insulating binders, or as dyes.

1 Claim, No Drawings POLYRHODANINE PHOTOCONDUCTIVE MATERIALS BACKGROUNDOF THE INVENTION SUMMARY OF AN EMBODIMENT OF THE INVENTION In oneembodiment, the invention comprises new compositions of mattercomprising polyrhodanine dyes having three or more rings therein. Thesepolyrhodanine dyes can be coated upon suitable electrically conductingsubstrates with or without binders to form photoconductive coatings. Onesuitable insulating binder is polystyrene.

Whereas most known organic photoconductors are colorless compounds andrequire added sensitizing dyes for sensitivity to visible light, thepolyrhodanine dyes described herein are colored. Therefore, they aresensitive to visible light without added sensitizers. Also, thepolyrhodanine photoconductors described herein have high sensitivitiesand low dark decay.

DESCRIPTION OF THE INVENTION The polyrhodanines described herein areformed by reacting a rhodanine, including N-alkyl or aryl rhodanines,with an ester such as methyl-p-toluene sulfonate at elevatedtemperatures. The reaction proceeds stepwise, the quaternary salt beingformed first reacting with another molecule of rhodanine followed byquaternization of this dinuclear product which then further condensesgiving a series of products.

The polyrhodanine compositions of this invention can be represented bythe following structural formula:

0 R o R o R ll l l l ll l 'l l \T I l H c c=o s s s wherein:

n equals 1, 2, 3 or 4; and,

R represents hydrogen; C -C alkyl including unsubstituted, substitutedand/or unsaturated alkyls; aryl; or aralkyl.

More specifically, these light-sensitive compositions can be representedby the following structural formulas:

R CO R N/ co R H2C c=c I N Ci I s c=o s S Polyrhodaninc I co R \ co /RCO R N me I N co C=C I N c=c l N S l s c=o S S Polyrhodanine II co R Nco R H ,C l N co R c=c I N co R c=c I N s l N s I s c=c s sPolyrhodanine Ill co R N CO R HZC N co R l \N' co R S C=C N l s c=c l NS c=c i Polyrhodanine IV One use for some of the polyrhodanine dyesdescribed and claimed herein is as photoconductive compositions. Forexample, polyrhodanine compositions having five or six rings have beenfound to be photoconductive. Such photoconductive compositions areuseful in preparing electrophotographic elements.

In preparing electrophotographic elements using polyrhodaninephotoconductors of this invention, the dye compositions may beformulated and coated with or without a binder. When a binder isemployed, the compound is dispersed in a solution of binder and solventand then after thorough mixing, the composition is coated on anelectrically conducting support in a well known manner such as swirling,spraying, doctor blade coating, etc. By electrically conducting, ismeant a resistivity of about 10 ohm-centimeters or lower, and preferablyof about 10 ohm-centimeters or lower.

Suitable binders comprise polymers having fairly high dielectricstrength and good electrically insulating characteristics. Byelectrically insulating," is meant a resistivity of ohm-centimeters orhigher, and preferably of about 10 ohm-centimeters or higher.

Examples of specific suitable binder materials in clude:styrene-butadiene copolymers; silicone resins; styrene-alkyd resins;silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride; acrylonitrile copolymers;poly(vinyl acetate); vinyl acetate; vinyl chloride copolymers;poly(vinyl acetals), such as poly(vinyl butyral); polyacrylic andmethacrylic esters, such as poly(methylmethacrylate),poly(n-butylmethacrylate), poly(isobutyl methacrylate), etc.;polystyrene; nitrated polystyrene; polymethylstyrene; isobutylenepolymers; polyesters, such as poly(ethylenealkaryloxyalkyleneterephthalate); phenol-formaldehyde resins; ketone resins; polyamide;polycarbonates; etc. Methods of making resins of this type have beendescribed in the prior art, for example, styrene-alkyl resins can beprepared according to the method described in U.S. Pat. Nos. 2,361,019and 2,258,423. Suitable resins of the type contemplated for uses in thephotoconductive layers of the invention are sold under such trade namesas Vitel PE- 101, Cymac, Piccopale 100, and Saran F-220. Other types ofbinders which can be used in the photoconductive layers of the inventioninclude such materials as paraffin, mineral waxes, etc.

Solvents of choice for preparing coating compositions of the presentinvention can include a number of solvents such as benzene, toluene,acetone, 2- butanone, chlorinated hydrocarbons, e.g., methylenechloride, ethylene chloride, etc., ethers. e.g., tetrahydrofuran, ormixtures of these solvents, etc.

Where a binder is used, it is satisfactory to have pigment/binder ratiosof from 2/1 to about 1/4. Preferably, the pigment/binder ratio is fromabout 1/1 to about U2.

The coating thickness of polyrhodanine dyes in solution can vary widely.In general, thicknesses, before drying, from about 0.0005 to about 0.01inches should be suitable. Preferably, the wet coating thicknesses arefrom about 0.0015 to about 0.006 inches.

Suitable supporting materials for coating the photoconductive layers ofthe present invention can include any of the electrically conductingsupports, for example, paper (at a relative humidity above 20%);aluminum-paper laminates; metal foils such as aluminum foil, zinc foil,etc.; metal plates, such as aluminum, copper, zinc, brass and galvanizedplates; regenerated cellulose and cellulose derivatives; certainpolyesters, es-

pecially polyesters having a thin electroconductive layer (e.g., cuprousiodide) coated thereon; etc. Suitable supporting materials can alsoinclude the humidity-independent conducting layers of semiconductorsdispersed in polymeric binders, as described in U.S. Pat. No. 3,112,192.

The elements of the present invention can be employed in any of the wellknown electrophotographic processes which require photoconductivelayers. One such process is the xerographic process. In a process ofthis type, the electrophotographic element is given a blanketelectrostatic charge by placing the same under a corona discharge whichserves to give a uniform charge to the surface of the photoconductivelayer. This charge is retained by the layer owing to the substantialinsulating property of the layer, i.e., the low conductivity of thelayer in the dark. The electrostatic charge formed on the surface of thephotoconducting layer is then selectively dissipated from the surface ofthe layer by exposure to light through an image-bearing transparency bya conventional exposure operation such as, for example, bycontact-printing technique, or by lens projection of an image, etc., toform a latent image in the photoconducting layer. By exposure of thesurface in this manner, a charge pattern is created by virtue of thefact that light causes the charge to be conducted away in proportion tothe intensity of the illumination in a particular area. The chargepattern remaining after exposure is then developed, i.e., renderedvisible, by treatment with a medium comprising electrostaticallyattractable particles having optical density. The developingelectrostatically attractable particles can be in the form of a dust,e.g., powder, a pigment in a resinous carrier, i.e., toner, or a liquiddeveloper may be used in which the developing particles are carried inan electrically insulating liquid carrier. Methods of development ofthis type are widely known and have been described in the patentliterature in such patents, for example, as U.S. Pat. No. 2,297,691, andin Australian Pat. No. 212,315. In processes of electrophotographicreproduction such as in xerography, by selecting a developing particlewhich has as one of its components, a low-melting resin, it is possibleto treat the developed photoconductive material with heat and cause thepowder to adhere permanently to the surface of the photoconductivelayer. In other cases, a transfer of the image formed on thephotoconductive layer can be made to a second support, which would thenbecome the final print. Techniques of the type indicated are well knownin the art and have been described in a number of U.S. and foreignpatents, such as U.S. Pat. Nos. 2,297,691 and 2,551,582, and in RCAReview, vol. 15, 1954, pp. 469-484.

Also, these polyrhodanines are dyes, usually from yellow to blue incolor. As such, they can be used, for example, to dye fibers, or tospectrally sensitize photosensitive systems to visible light.

The polyrhodanines described herein can also be used as dopants to raisethe sensitivity of lower sensitivity photoconductors. This use isdescribed in more detail in my copending application, Ser. No. 366,906,filed June 4, 1973, now U.S. Pat. No. 3,877,937. These polyrhodanineshave particularly beneficial results when small amounts are intimatelydispersed with organic, pigment-type photoconductors as described in mycopending application, Ser. No. 366,907, filed June 4, 1973. Theteachings from both of my abovementioned copending applications arehereby expressly incorporated by reference.

The following examples further illustrate the inven tion.

EXAMPLE 1 Preparation of Polyrhodanine I Ten grams of N-ethylrhodaninewere mixed with 20 grams of methyl-t-toluenesulfonate and heated to150C. After reaching 150C, the reaction mixture turned darker rapidlyand the temperature rose to 200C. without further application of heat.The reaction mixture was left standing for minutes, during which timethe temperature fell to 160C. The mixture was further cooled to 20C.,stirred with ether, decanted and treated with several further portionsof ether until a heavy, sticky residue remained. This residue wasstirred with 350 ccs. of absolute ethanol, heated and left standingovernight at room temperature. The next morning the solution wasfiltered with suction and the solid product was washed with ethanol. 4.2grams of a solid product were obtained; this product was a mixture ofpolyrhodanine compounds I, ll,'llI and IV.

The ethanol filtrate was concentrated to 150 ccs. at which time crystalsstarted to form in the boiling solution. The mixture was cooled and theproduct filtered off to yield 2.1 grams of yellow crystals, with anabsorption maximum in Cresol at 450 nm. The crystals were extracted with7x200 ccs. of boiling methanol. The extracts were combined and chilled.Crystals formed and were filtered off. A small amount of an impurity,having an absorption maximum at 494 nm., was removed by dissolving in300 ccs. in boiling acetone and treatment with four consecutive portionsof Norite A. The acetone solution was concentrated to 50 cc. and cooled.On filtration, Polyrhodanine I was obtained as yelloworange crystals.M.P. 269271C. D-max: 1.24 in pyridine, l/l10,000 at 424 nm. (430 inCresol).

EXAMPLE 2 I Preparation of Polyrhodanine II The 4.2 grams of originalcrystal product in Example 1 were extracted with 4X50 ccs. of hotacetone to remove most of Polyrhodanine I which was still present inthis mixture, as shown by a curve, which indicated absorption maxima incresol at 430, 495, 550 and 610 nm. 3.8 grams remained undissolved, ofwhich 3.5 grams were extracted with 250 ccs. of boiling pyridine. Thesolution was cooled and the crystals that formed were filtered off toyield 0.8 grams of Polyrhodanine II. D-max: 0.78 in pyridine, l/210,000at 470 nm. (495 in Cresol EXAMPLE 3 Preparation of Polyrhodanine Ill Thesolid material remaining after extraction with boiling pyridine inExample 2 was further extracted with 4X50 ccs. of boiling pyridine. Onlya very small amount went into solution. After standing overnight in therefrigerator, the solution was filtered, and 0.015 grams of a lightpurple dye, polyrhodanine III, was obtained having an absorption maximumat 550 nm. in

Cresol.

EXAMPLE 4 Preparation of Polyrhodanine IV 0.5 grams of the materialstill undissolved after Example 3 was extracted with 50 ccs. of Cresolat 150l60C. The remaining residue was stirred with acetone and filtered,and then thoroughly washed with acetone to remove all the Cresol. 0.2grams polyrhoda- 5 nine IV were obtianed as a dark purple material,having an absorption maximum at 610 nm. in Cresol.

EXAMPLES 5 and 6 Preparation of Electrophotographic PlatesElectrophotographic plates containing polyrhodanine pigment-typephotoconductors as described herein were prepared as follows.

50 mg. of pigment was dispersed in one-half cc. of a 10% solution ofpolystyrene in tetrahydrofuran. This solution was coated onto a 0.006inch grained aluminnm plate by means of a Bird applicator to give a0.003 inch wet thickness. The plate was dried for a minimum of 30minutes at 100C. before testing for dark decay and sensitivity on anelectrostatic testing apparatus. This apparatus rotated the plate undera corona charger until the surface potential (measured on anoscilloscope) reached 500 volts. The charging current was shut off andthe percent drop in voltage after seconds was recorded (dark decay). Theplate was then recharged to 500 volts and exposed to a 15 watt tungstenlight source held 2 inches from the sample. Percent drop in voltage in 3seconds was recorded as sensitivity. 1f 100% discharge, or amountsapproaching 100% occurred in less than 3 seconds, the charge exposurecycle was repeated using a 0.5, 1.0 or 2.0 neutral density filterbetween the sample and the light source.

The following table presents dark decay and sensitivity data for variouspolyrhodanine photoconductors, determined according to the aboveprocedure.

TABLE Ex. Polyrhodanine No. Photoconductor Dark Decay Sensitivity 5 Ill15 100-2 sec.

30(l) 6 IV 70 lOO-l sec.

"1.() neutral density filter What is claimed is: 1. A photoconductivecomposition comprising a photoconductive polyrhodanine dispersed in anelectrically insulating binder and having a pigment to binder ratio offrom about 2/1 to about 1/4, said photoconductive polyrhodanine beingrepresented by the following structural formula:

O R O R O R l 1 l1 1 l N ("N HA 2 1 (i=0 S S S wherein:

n equal 1, 2, 3 or 4; and, R represents hydrogen; C C alkyl; aryl; oraralkyl.

1. A PHOTOCONDUCTIVE COMPOSITION COMPRISING A PHOTOCONDUCTIVEPOLYRHODANINE DISPERSED IN AN ELECTRICALLY INSULATING BINDER AND HAVINGA PIGMENT TO BINDER RATIO OF FROM ABOUT 2/1 TO ABOUT 1/4, SAIDPHOTOCONDUCTIVE POLYRHODANINE BEING REPRESENTED BY THE FOLLOWINGSTRUCTURAL FORMULA: